1. Field of the Invention
This invention relates to a solenoid valve for hydraulic control, and more particularly to improvements in a solenoid valve for hydraulic control suitable for being assembled into a hydraulic control device in a vehicle automatic transmission. The solenoid valve is light in weight, compact in size and high speed in response.
2. Description of the Prior Art
In general, in hydraulic control devices solenoid valves have been used to allow flow and to cut off oil flow in specific oil lines, or, they have been combined with orifices which create or reduce hydraulic pressure in specific oil lines.
FIG. 3 shows an example of a conventional solenoid valve of this type. In FIG. 3 10 designates a stationary iron core, 12 a coil, 14 a plunger which additionally functions as a valve body, 16 an input port and 18 an output port. This solenoid valve is constructed such that the plunger 14 thereof is center-aligned by the inner peripheral portion of a guide pipe 20 made of a non-magnetic material and slidable on the inner peripheral portion of the guide pipe 20.
Another conventional solenoid valve is shown in FIG. 4. In FIG. 4, 10A designates a stationary iron core, 12A a coil, 14A a plunger additionally functioning as a valve body, 16A an input port and 18A an output port, respectively.
This solenoid valve is constructed such that the plunger 14A is comprised of a large-diameter land 14A.sub.1 disposed in a columnar recess 14A, of the stationary iron core 10A and a small-diameter land 14A.sub.2 coaxially and integrally formed on the large-diameter land 14A.sub.1. The outer peripheral surface of the large-diameter land 14A.sub.1 faces the inner peripheral surface of the columnar recess 10A.sub.1 of the stationary iron core 10A, and the outer peripheral portion of the small-diameter land 14A.sub.2 is slidable on guide pipe 20A.
However, in the conventional solenoid valve shown in FIG. 3, when a current is passed to the coil 12, a magnetic path is formed as indicated by an arrow A. In this case, since the wall thickness of the guide pipe 20 cannot be decreased very much from the viewpoint of durability, a disadvantage has been the viewpoint of durability, a disadvantage has been presented in that a magnetic reluctance of the magnetic path is increased and a plunger attracting force is decreased while the current is passed to the coil. This causes the time duration for response, i.e. the time duration required for opening the solenoid valve, to be lengthened. Furthermore, an additional disadvantage is that, when the current passage to the coil is stopped and the plunger 14, additionally functioning as the valve body, is about to close a valve seat 22 through the resiliency of a return spring 15, the solenoid valve receives a reaction force of a high hydraulic pressure from the input port 16, whereby the time duration required for closing the solenoid valve is increased. This disadvantage can be obviated by making the whole solenoid valve larger in size to obtain a higher attracting force. However, this leads to an another disadvantage in that the solenoid valve is increased in weight, the space required for mounting the solenoid valve is increased and the production cost is increased.
On the other hand, with the conventional solenoid valve shown in FIG. 4, when current is passed to the coil 12A, a magnetic path indicated by an arrow B is formed, whereby a magnetic reluctance is relatively low, so that the disadvantageous lowering of the attracting force due to an increase in magnetic reluctance is obviated. Furthermore, since the input port 16A is disposed in a direction perpendicular to the sliding direction of the plunger 14A, the disadvantage of the valve closing not being smooth due to the high hydraulic pressure from the input port 16A is obviated. However, this structure is disadvantageous because the plunger 14A is constructed such that the large-diameter land 14A.sub.1 partially forming the magnetic path B is integrally formed with the small-diameter land 14A.sub.2, additionally functioning as the valve body, whereby the mass of the movable portions is large. Thus, the attracting force is relatively decreased, creating the disadvantage that high speed response cannot be obtained in the time durations required for both the opening and closing the solenoid valve. Furthermore, in order to center-align the large heavy plunger 14A with high accuracy, it is necessary to enlarge the guide pipe 20A which is constructed of non-magnetic material, the result is that an outer diameter D of the valve seat portion is disadvantageously increased. Further since the input port 16A of the hydraulic pressure is provided at the side portion of the solenoid valve (upwardly in the drawing) and the output port 18A is disposed on the axis, disadvantages are presented in that a counterpart hydraulic control component, to which this solenoid valve is mounted, becomes complicated in circuit and in positional arrangement, and the space required for mounting the solenoid valve disadvantageously tends to increase.